Alterations in the heart that occur during the aging process result in decreased myocardial function and render it more susceptible to damage. A common cause of damage to the myocardium is ischemic injury. Until now, experimental evidence linking a decline in ischemic stress tolerance to alterations in specific stress signaling pathways has been lacking. Recently, we have found that the AMP-activated protein kinase (AMPK) signaling pathway plays an important role in limiting cardiac necrosis, apoptosis and dysfunction induced by ischemia/reperfusion, and aging-associated reduction in AMPK activity that may be an important contributing factor in the reduced mitochondrial function and dysregulated intracellular lipid metabolism associated with aging in skeletal muscles. The applicant hypothesizes that aging is associated with a decline in the ability of cardiac cells to activate this host response to acute ischemic injury, which contributes to a reduced tolerance to ischemic insults. We will test this hypothesis by addressing the following specific aims using hearts from young (4-6 months of age) and old (24-26 months of age) Fisher 344 rats and C57BL/6 mice: i) to examine whether aged heart displays impaired activation of upstream mediators of AMPK stress signaling by clinically important ischemia/reperfusion stress and to investigate the mechanisms responsible for this alteration in signaling. ii) to determine the effects of aging on the multifactorial downstream effectors of stress-induced AMPK signaling pathways in the ischemic heart, and to elucidate the role of stress signaling in impaired ischemic tolerance in aging. This aim will address whether defective AMPK activation with aging leads to reduced downstream signaling and increased susceptibility of the myocardium to ischemia/reperfusion injury. AMPK deficient (KD) transgenic mice and pharmacologic stimulation of AMPK will be used to demonstrate the important role of AMPK in maintaining ischemic tolerance. We also outline future experiments to examine the effect of modulating behaviors, namely caloric intake and exercise, on the AMPK signaling pathway in the aged heart, to elucidate the potential role of interventions that might lead to AMPK stimulation in managing cardiovascular disease in the elderly. Better understanding of the mechanisms leading to altered cardiac AMPK activation in response to ischemic stress with aging is important to fully understand the basis for increased susceptibility of the elderly to ischemic injury and could lead to therapeutic strategies aimed at limiting cardiac damage.